Proposals have already been made to use the Compton effect to determine the position of a separation plane between two media of different densities. Thus, in an article entitled "Compton backscatter tomography of low atomic number materials with the ComScan system", published in January 1989 by J. Kosanetzky, G. Harding, K. H. Fisher, and A. Meyer, of Philips Forschungslaboratorium Hamburg, the principle of using the Compton effect has already been described, together with a device for performing tomography on a body by subjecting it to X-rays or to gamma rays, which rays produce omnidirectional radiation representative of the density of the irradiated volume of said body by interacting with the electrons in the atoms of the body. An emitter of radiation produces a relatively fine pencil that passes through the body under investigation, and the Compton effect radiation therefrom is detected by receivers. A mechanism enables the apparatus to be displaced as a whole perpendicularly to the surface of the body through which the radiation penetrates, thereby taking measurements at various depths beneath said surface.
However, such a apparatus suffers from the drawback of using a plurality of emitters of radiation and a plurality of associated pairs of detectors. The detectors are also large in area with only a small portion thereof being in use at any given instant, such that their unused portions give rise to background noise in the electronic detection system and, in addition, receive Compton effect emissions that have been subjected to scatters on a plurality of occasions in volumes of the body that are not directly irradiated. Taken together, these interfering phenomena limit the accuracy of measurement.
The present invention seeks to obviate these drawbacks by providing a method and apparatus that improve the spatial resolution of measurements enabling the plane between two media of different densities to be located.